Hmm, I use 670rpm all the time. Carved .375" x 5" off a 1" bar of EN8 this evening, 3 bites + a finishing cut, gotta love a strong lathe... The chips were nearly black on one side, you could really smell the sulpher in the oil working. The only trouble, in fact, with going that fast is watching out for work hardening...
I was keeping an eye on oxford oil filled welders for a long while on ebay, hoping to find one for sale up north.
A welder mate of mine has a big oil-filled arc welder he never uses, he'd almost certainly part for it for scrap money. Based in Wallasey (CH44) if that's any use to you. It'll be a hulking great 3-phase thing, I have no idea what make, or model, can get some details if you're interested.